1. Field of the Invention
This invention relates to an information-discriminating circuit which is suitable for use as an interface of ISDN (Integrated Services Digital Network).
2. Prior Art
As information-oriented society has advanced in these days, it is demanded to transmit a large amount of information. ISDN is provided to meet such demand, and the residence of a subscriber thereof is normally provided with two 64-kbps channels and a 16-kbps channel. In such a case, information is transmitted in the form of a ternary signal over telecommunication lines such as a telephone line. Therefore, a receiving side, such as facsimile terminal equipment and a MODEM, has to be equipped with an information-discriminating circuit for discriminating information in the ternary signal transmitted thereto.
The ternary signal supplied to the receiving side is transmitted through various transmission lines, such as a telephone line, relay stations, and terminal equipment, and therefore the signal input to the receiving side varies in amplitude depending on characteristics of the transmission lines. If the input signal thus varying in amplitude is compared with a fixed threshold value to discriminate information in the signal, a sufficient noise margin cannot be obtained, to thereby increase the bit error rate. To overcome this inconvenience, the conventional information-discriminating circuit changes the threshold value according to the amplitude of the input signal.
The conventional information-discriminating circuit will now be described hereinbelow with reference to FIGS. 1 to 3.
FIG. 1 shows the circuit arrangement of the conventional information-discriminating circuit. In the figure, a transformer T has an output terminal t3 on a secondary side thereof, which is connected to a junction P3 between resistances 13 and 14. Resistances 11, 12, 15, and 16, and the resistances 13 and 14 are serially connected between a power supply and ground, to divide supply voltage. The values of the resistances 11 to 16 are set so as to be symmetrical with respect to the junction P3 between the resistances 13 and 14. More specifically, the values of the resistances 11 and 16 are set to a value R1, the values of the resistances 12 and 15 to a value R2, and the values of the resistances 13 and 14 to a value R3, respectively. A capacitor C is connected between the output terminal t3 and ground and functions as a bypass capacitor for eliminating an AC component of a signal input thereto to stabilize the voltage. A selector 21 has input terminals A and B thereof connected to junctions P1 and P2 of the resistances 11, 12 and 13, and a selector 21 has input terminals A and B to junctions P4 and P5 of the resistances 14, 15 and 16, respectively. In the illustrated example, voltage at the input terminal A of the selector 21 is equal to Vc+V2, and voltage at the input terminal A of the selector 22 is equal to Vc-V2. On the other hand, voltage at the input terminal B of the selector 21 is equal to Vc+V3, and voltage at the input terminal B of the selector 22 is equal to Vc-V3.
When an input signal Vin is supplied to input terminals t1 and t2 of the transformer T, a DC component of the input signal Vin is eliminated by the transformer T, and the resulting input signal Vin is supplied from an output terminal t4 of the transformer T to an input level-detecting circuit 30. The input level-detecting circuit 30 detects the amplitude of the input signal Vin (hereinafter referred to as "the input level"), and compares the detected amplitude with a predetermined value, to thereby generate a control signal which controls the selectors 21 and 22. In the illustrated example, when the input level exceeds the predetermined value, the voltage at the input terminal A of the selector 21 or 22 is selected, whereas when the input level is below the predetermined value, the voltage at the input terminal B of the selector 21 or 22 is selected. Further, output voltage from the selector 21 is applied to a negative input terminal of a comparator 41, and output voltage from the selector 22 to a positive input terminal of a comparator 42, respectively, the both output voltage values being used as threshold values for discriminating information in the input signal Vin.
Next, description will be made of how the conventional information-discriminating circuit operates when the input level is so low that it is below the predetermined value, with reference to a timing chart of FIGS. 2A to 2C. FIG. 2A shows a waveform of the input signal Vin supplied to the comparators 41 and 42, which fluctuates with respect to the voltage Vc as a central value. In this case, the threshold values of the comparators 41 and 42 are set to Vc+V3 and Vc-V3, respectively. As a result, the comparators 41 and 42 generate output voltages V(+) and V(-) as shown in FIGS. 2B and 2C, respectively.
Further, description will be made of how the circuit operates when the input level is so high that it exceeds the predetermined value, with reference to a timing chart of FIGS. 3A to 3C. FIG. 3A shows a waveform of the input signal Vin supplied to the comparators 41 and 42, which fluctuates with respect to the voltage or central value Vc. In this case, the threshold values of the comparators 41 and 42 are set to Vc+V2 and Vc-V2, respectively. As a result, the comparators 41 and 42 generate output voltages V(+) and V(-) as shown in FIGS. 3B and 3C, respectively.
As described above, the conventional information-discriminating circuit stepwise changes the threshold values according to the input level to increase the noise margin.
In the conventional information-discriminating circuit, the threshold values of the comparators 41 and 42 are set by the use of the resistances 11 to 16. Therefore, the threshold values can be only stepwise set even if the number of the resistances is increased. Thus, the conventional information-discriminating circuit has the disadvantage that the maximum noise margin cannot be always obtained.
To increase the noise margin, the number of threshold values needs to be increased. If the number of threshold values is increased, however, the process of generating the control signal by the input level-detecting circuit 30 becomes complicated, and further the number of input terminals of the selectors 21 and 22 must be increased, which unfavorably increases the number of component parts of the information-discriminating circuit.